MCAT Organic Chemistry Review



This chapter was full of numbers and values, but the most important thing to know about spectroscopy on the MCAT is that you don’t need to know a lot of numbers. The numbers that you do need to know have already been stressed heavily in this chapter. Know that infrared (IR) spectroscopy is best for identifying the presence (or, more importantly, the absence) of functional groups. A cursory understanding of ultraviolet (UV) spectroscopy and its association with conjugation will suffice. Nuclear magnetic resonance (NMR) spectroscopy—specifically, proton (1H) NMR— also helps us figure out the arrangement of functional groups. Know how to interpret IR and NMR spectra: IR spectra have three important peaks (O–H, C=O, and N–H), but NMR spectra can be far more complex. The MCAT can test the chemical shift of deshielded protons, which will be downfield, or toward the left of the spectrum. Make sure that you can interpret peak splitting, which is due to interference from neighboring hydrogens, and peak integration, which is proportional to the number of magnetically identical hydrogens.

Spectroscopy is often tested on the MCAT in the context of experiment-based passages. As you continue studying the reaction chemistry discussed in Chapters 4 through 10 of MCAT Organic Chemistry Review, consider what these products would yield in different spectroscopic modalities.

This chapter focused on one method of identifying compounds based on structural characteristics and interactions with electromagnetic energy, but spectroscopy is not the only method for characterizing organic molecules. In the next chapter, we explore another side of laboratory techniques: separation and purification schemes. These utilize chemical differences between molecules to allow us to isolate and describe them.

Concept Summary

Infrared Spectroscopy

·        Infrared (IRspectroscopy measures absorption of infrared light, which causes molecular vibration (stretching, bending, twisting, and folding).

·        IR spectra are generally plotted as percent transmittance vswavenumber 

o   The normal range of a spectrum is 4000 to 400 cm–1.

o   The fingerprint region is between 1500 and 400 cm–1. It contains a number of peaks that can be used by experts to identify a compound.

·        To appear on an IR spectrum, vibration of a bond must change the bond dipole moment. Certain bonds have characteristic absorption frequencies, which allow us to infer the presence (or absence) of particular functional groups.

o   The O–H peak is a broad peak around 3300 cm–1. Molecules with O–H include alcohols, water, and carboxylic acids; the carboxylic acid O–H peak will be around 3000 cm–1.

o   The N–H peak is a sharp peak around 3300 cm–1. Molecules with N–H include some amines, imines, and amides.

o   The C=O peak is a sharp peak around 1750 cm–1. Molecules with C=O include aldehydes, ketones, carboxylic acids, amides, esters, and anhydrides.

Ultraviolet Spectroscopy

·        Ultraviolet (UV) spectroscopy measures absorption of ultraviolet light, which causes movement of electrons between molecular orbitals.

·        UV spectra are generally plotted as percent transmittance or absorbance vs. wavelength.

·        To appear on a UV spectrum, a molecule must have a small enough energy difference between its highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO) to permit an electron to move from one orbital to the other.

o   The smaller the difference between HOMO and LUMO, the longer the wavelengths a molecule can absorb.

o   Conjugation occurs in molecules with unhybridized p-orbitals. Conjugation shifts the absorption spectrum to higher maximum wavelengths (lower frequencies).

Nuclear Magnetic Resonance Spectroscopy

·        Nuclear magnetic resonance (NMRspectroscopy measures alignment of nuclear spin with an applied magnetic field, which depends on the magnetic environment of the nucleus itself. It is useful for determining the structure (connectivity) of a compound, including functional groups.

o   Nuclei may be in the lower-energy α-state or higher-energy β-state; radiofrequency pulses push the nucleus from the α-state to the β-state, and these frequencies can be measured.

·        Magnetic resonance imaging is a medical application of NMR spectroscopy.

·        NMR spectra are generally plotted as frequency vs. absorption of energy. They are standardized by using chemical shift (δ), measured in parts per million (ppm) of spectrophotometer frequency.

o   NMR spectra are calibrated using tetramethylsilane (TMS), which has a chemical shift of 0 ppm.

o   Higher chemical shifts are located to the left (downfield); lower chemical shifts are located to the right (upfield).

·        Proton (1H) NMR is the most common.

o   Each unique group of protons has its own peak.

o   The integration (area under the curve) of this peak is proportional to the number of protons contained under the peak.

o   Deshielding of protons occurs when electron-withdrawing groups pull electron density away from the nucleus, allowing it to be more easily affected by the magnetic field. Deshielding moves a peak further downfield.

o   When hydrogens are on adjacent atoms, they interfere with each other’s magnetic environment, causing spin–spin coupling (splitting). A proton’s (or group of protons’) peak is split into n + 1 subpeaks, where n is the number of protons that are three bonds away from the proton of interest.

o   Splitting patterns include doubletstriplets, and multiplets.

o   Protons on sp3-hybridized carbons are usually in the 0 to 3 ppm range (but higher if electron-withdrawing groups are present). Protons on sp2-hybridized carbons are usually in the 4.6 to 6.0 ppm range. Protons on sp-hybridized carbons are usually in the 2.0 to 3.0 ppm range.

o   Aldehydic hydrogens tend to appear between 9 to 10 ppm.

o   Carboxylic acid hydrogens tend to appear between 10.5 and 12 ppm.

o   Aromatic hydrogens tend to appear between 6.0 and 8.5 ppm.

Answers to Concept Checks

·        11.1

1.    IR spectroscopy measures absorption of infrared light by specific bonds, which vibrate. These vibrations cause changes in the dipole moment of the molecule that can be measured. Once the bonds in a molecule are determined, one can infer the presence of a number of functional groups to determine the identity of the molecule.

2.    A carboxylic acid would have a broad O–H peak around 2800–3200 cm–1 and a sharp carbonyl peak at 1700–1750 cm–1.

·        11.2

1.    Molecules with π or nonbonding electrons, and conjugated systems, will give absorbances on a UV spectroscopy plot.

2.    HOMO is the highest occupied molecular orbital; LUMO is the lowest unoccupied molecular orbital. The smaller the difference in energy between the two, the longer the wavelengths that can be absorbed by the molecule.

·        11.3

1.    NMR measures alignment of the spin of a nucleus with an applied magnetic field. It is most often used for identifying the different types and magnetic environments of protons in a molecule, which allows us to infer the connectivity (backbone) of a molecule.

2.    The units for chemical shift with a standardized NMR spectrum are parts per million (ppm).

3.    Deshielding occurs in molecules that have electronegative atoms that pull electron density away from the hydrogens being measured. This results in a downfield (leftward) shift of the proton peak.

4.    Spin–spin coupling occurs when two protons close to one another have an effect on the other’s magnetic environment. This results in the splitting of peaks into doublets, triplets, or multiplets, depending on the environment.

Shared Concepts

·        General Chemistry Chapter 1

o   Atomic Structure

·        General Chemistry Chapter 3

o   Bonding and Chemical Interactions

·        Organic Chemistry Chapter 3

o   Bonding

·        Organic Chemistry Chapter 12

o   Separations and Purifications

·        Physics and Math Chapter 8

o   Light and Optics

·        Physics and Math Chapter 9

o   Atomic and Nuclear Phenomena